How do we protect the astronauts from space radiation?

A number of parameters affect astronaut exposure to radiation. These parameters include the structure of the spacecraft, the materials used to construct the vehicle, the altitude and inclination of the spacecraft, the status of outer zone electron belts, the interplanetary proton flux, geomagnetic field conditions, solar cycle position, and EVA start time and duration. SRAG considers all of these parameters in order to ensure that radiation exposures received by the astronauts remain below established safety limits. Specific components of this responsibility include:

The radiation consoles in the Mission Control Center at Johnson Space Center are staffed four hours daily during nominal space weather conditions, and continuously during extra-vehicular activities (EVA's) and significant space weather activity. SRAG works very closely with the Space Environment Center (SEC) at the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado. NOAA continuously monitors data received from its space weather satellites and ground stations to provide current information and forecasts about the space environment. Solar Forecasters at the SEC provide round the clock support, providing alerts and warnings about space weather conditions by telephone and pager and by displaying real time operational space weather data via the Internet.

SRAG's Multi-Purpose Support Room in the Mission Control Center at the Johnson Space Center, Houston, Texas

NOAA's Space Environment Center, Boulder Colorado

During nominal conditions, SRAG team members examine NOAA's space weather data, reports, and forecasts for trends or conditions which may produce enhancements to the near-Earth radiation environment; they then report the information to flight management. They also verify that the ALARA (As Low As Reasonably Achievable) principle is maintained. When space weather reports indicate that conditions for significant change are present (even though an increase may not occur), the radiation consoles are manned for longer periods, and the team ensures that radiation monitoring hardware is fully functional. They may also recommend restricting or rescheduling of EVAs. When there is a confirmed increase in radiation exposure levels, SRAG monitoring operations in Mission Control increase to 24 hours per day. During such times, SRAG team members may recommend active seeking of shielded areas inside the spacecraft, and either the cancellation or revised scheduling of EVAs.

In order to support mission planning, the Space Radiation Analysis Group maintains an extensive set of tools for estimating the exposure received by the crews of manned missions in Low Earth Orbit (LEO). This suite of tools includes time-resolved models of the Earth's magnetic field, maps of the radiation fluxes trapped in the geomagnetosphere, and trajectory translator/propagator algorithms. Space environment conditions (interplanetary proton flux, status of the electron belts, geomagnetic field conditions) are integrated with mission parameters (altitude and inclination of the spacecraft, location and timing of EVAs) in order to project crew exposures.

Low inclination, high altitude flights during solar minimum produce higher dose rates than those with high inclination, low altitude flights during solar maximum. At higher altitudes, the area of the South Atlantic Anomaly is larger and the concentration of protons is higher. Although trajectories of high inclination flights pass through the regions of maximum intensities within the South Atlantic Anomaly, less time is spent there than for low inclination flights, and crews on high inclination flights typically receive less net exposure to trapped radiation for the same altitude. During solar maximum, increases in the Sun's activity expand the atmosphere; this expansion causes losses of some of the protons in the radiation belts due to interactions with atmospheric gases. Therefore, trapped radiation doses decrease during solar maximum and increase during solar minimum. The impact of galactic cosmic radiation is also lower during solar maximum because the increased speed and density of the solar wind increases the interplanetary magnetic field generated by the Sun. This strengthening of the Sun's magnetic field makes it more difficult for GCRs to penetrate the inner solar system in a process similar to that of fish swimming upstream.

Any request to fly a radioactive isotope or piece of radiation producing equipment must be submitted to the JSC Radiation Constraints Panel, Radioactive Payloads Working Group for approval. For each isotope or piece of equipment, the amount of radiation produced is evaluated and internal and external dose rates that the astronauts might receive as a result of their proximity are calculated. Additionally, containment and decontamination procedures are examined to verify that the use of these items conforms to the ALARA principle. SRAG team members serve as members of this panel, and they are responsible for evaluating and authorizing these requests.

SRAG's modeling tools include state-of-the-art radiation transport codes and CAD-based geometry evaluation tools. By using these tools as part of an information feedback loop and by using real measurements to continuously refine the process, SRAG is able not only to react to the on-orbit environment, but also to anticipate situations and act to preclude contingencies before they occur.

The suite of instruments used to monitor the radiation environment include the Tissue Equivalent Proportional Counter (TEPC), the Charged Particle Directional Spectrometer (CPDS), the Radiation Area Monitor (RAM), and the Crew Passive Dosimeter (CPD). All of these instruments are flown on both the International Space Station (ISS) and the Space Shuttle, with the exception of the CPDS, which is flown only on the ISS.

Tissue Equivalent Proportional Counter (TEPC)

ISS Tissue Equivalent Proportional Counter

The TEPC is designed to measure the dose that a small volume of tissue would receive from a wide variety of radiation sources. It simulates a 2µm diameter volume of tissue using a cylindrical detector design. The detector volume is 2 inches in diameter and 2 inches long, and is filled with a very low pressure of propane gas. The gas volume is surrounded by tissue equivalent plastic. The organic molecules in the plastic and gas effectively simulate the cell wall and cell body respectively.

When radiation interacts with the detector, electrons are produced and accelerated towards a small wire in the middle of the detector that is held at a high positive voltage. As the electrons accelerate towards the wire, other electrons are created, and an amplification of the initial event occurs. The electrons are collected by the wire and a signal pulse is generated that is proportional to the energy of the radiation that hit the detector. The signal pulses are then amplified and stored in memory in the spectrometer portion of the instrument until they are downloaded to the ground for detailed analysis.

TEPC Cumulative Dose & Dose Rate Display

The spectrometer also performs real-time calculations and displays the average dose rate and other parameters on a small LCD screen on the instrument for use by the astronauts, and sends similar information to Mission Control that allows SRAG personnel to constantly monitor the radiation environment inside the spacecraft. The ISS TEPC also has an alarm capability that will inform the Crew and ground personnel if radiation levels exceed a predetermined threshold. The ISS TEPC is also designed to be portable so that it can be moved around inside the spacecraft and the radiation environment inside the vehicle can be mapped.

Charged Particle Directional Spectrometer (CPDS)

CPDS Detector Stack Arrangement

The Charged Particle Directional Spectrometer instrument is designed to measure the charge, energy, and direction of a particle that passes through the instrument. There are 13 separate detectors inside the CPDS that are arranged in a stack. The three A-detectors are 1mm thick silicon dE/dx detectors. The A1 and A2 detectors are used in coincidence mode to record an event (passage of a charged particle through the instrument). The three PSD (Position Sensitive Detectors) are 0.3 mm thick silicon detectors, with the active area of the detector arranged in a series of 24 horizontal and 24 vertical 1 mm wide strips. This arrangement of x and y strips allows the instrument to record the x and y coordinate of a particle as it travels along the z-axis through the instrument. The three B-detectors are 5 mm thick lithium-drifted silicon dE/dx detectors. The final detector in the stack is the C-detector or Cerenkov detector. The Cerenkov detector consists of a 1 cm thick single crystal sapphire radiator combined with a photo-multiplier tube. The amount of light gathered by the photo-multiplier is a function of the particle velocity, and so this detector provides very different information than the energy deposition recorded by the dE/dx detectors. All thirteen detectors are arranged into a stack inside the instrument forming a telescope of sorts. Only particles passing through the cone of acceptance formed by the A1 and A2 detectors are recorded.

Currently, there are four CPDS instruments in use on-board the ISS. The first is the Intra-Vehicular Charged Particle Directional Spectrometer (IV-CPDS). The IV-CPDS is designed to be used inside the ISS with mounting and power options for both the US and Russian segments. The IV-CPDS also performs real-time calculations and displays the average dose rate and other parameters on a small LCD screen on the instrument for use by the astronauts, and sends similar information to Mission Control that allows SRAG personnel to constantly monitor the radiation environment inside the ISS. The remaining three CPDS instruments are mounted outside the ISS in the form of the Extra-Vehicular Charged Particle Directional Spectrometer (EV-CPDS). The EV-CPDS instruments are arranged such that one points forward along the velocity vector (EV1), one points aft along the anti-velocity vector (EV3), and the third points up along the zenith direction (EV2). The EV-CPDS instruments have no LCD displays and are coated with a special material to allow the instruments to survive the temperature extremes experienced in the vacuum of space.

IV-CPDS

EV-CPDS

Radiation Area Monitor (RAM) and Crew Passive Dosimeter (CPD)

Radiation Area Monitor

The Radiation Area Monitor (RAM) is a small set of thermoluminescent detectors (TLD) encased in a Lexan holder.

SRAG's Radiation Dosimetry Lab, JSC

The material responds to radiation via electronic excitation states in the various TLD materials.
After exposure, the amount of absorbed energy (dose) is determined by applying heat and measuring the amount of visible light released as these excited states are returned to equilibrium. RAMs are placed in throughout the volumes of both the ISS and the Space Shuttle; the ISS monitors are swapped out during the periodic Shuttle missions. The Crew Passive Dosimeter (CPD) is identical to the RAM and is carried by each member of the crew during the entire mission.

SRAG team members in the Radiation Dosimetry Lab at JSC prepare the RAM's and CPD's prior to flight and analyze them when they return. They submit reports containing the results of the analyses to the Space Radiation Health Officer and to the Crew Surgeon. These reports are retained as a permanent record of the crew members' health history, and may be used to determine eligibility for flight.